A comprehensive review of structural alterations in CO₂-interacted coal: insights into CO₂ sequestration in coal

The greenhouse gas effect, which primarily comes from the vast destruction of forests and energy production from burning fossil fuels including oil and coal, is driving up the Earth's temperature and fundamentally changing the world around us. In a portfolio of options which aid in the stabilization of greenhouse gases, especially the anthropogenic CO2 level, CO2 sequestration in coal reservoirs is a better option, as the process is associated with coalbed methane extraction, which offsets the cost of sequestration. A detailed understanding of coal's structural alterations which occur upon CO2 injection is imperative for productive and safe sequestration. This paper presents a comprehensive review of CO2 interaction-induced structural alterations in coal and the consequent hydro-mechanical alterations pertinent to CO2 sequestration, particularly coal permeability and strength. Further, numerical modeling approaches suitable for modeling the fully coupled processes and the underlying multiphysics for each process are discussed, with reference to previous analytical and numerical studies. CO2 interaction causes significant alterations in coal's macromolecular structure, mineral phase, natural fracture network, and matrix pore network. The plasticization effect, hydrocarbon mobilization, adsorption-induced matrix swelling and surface energy reduction, shrinkage- and differential swelling-induced microcracking, and the dissolution and precipitation of minerals may adversely affect reservoir permeability and mechanical parameters, imperilling the productivity of CO2 sequestration and reservoir integrity in general. Since the level of alterations depends on the CO2 interaction environment and the characteristics of the coal reservoir, they should be evaluated in relation to the targeted reservoir properties. In the context of numerical modeling, the approach should particularly consider the coupling between the hydraulic and mechanical parameters caused by the sorption-induced swelling and the effective stress change. Furthermore, the explicit representation of the fracture network in model geometry is important, because the alterations are highly localized and heterogeneous in a typical fractured coal reservoir.